Modification of surfaces to increase the surface tension

ABSTRACT

The invention concerns an analytical test element in which the sample liquid is transported from a sample application site to a determination site, where a detection site lies upstream of the sample application site in the transport direction. The analytical test element has at least one surface which is composed of at least one element that can be oxidized with water or an alloy that can be oxidized with water which has been treated by the action of boiling water or water vapour.

RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 11/374,332, filed Mar.13, 2006, which is a divisional of U.S. application Ser. No. 09/555,618,filed Jan. 5, 2001 and claims priority to PCT/EP98/07851 filed Dec. 3,1998 and German Patent Application No. 197 53 848.7, filed Dec. 4, 1997,which is each hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The invention concerns a process for the production of surface coatingsas well as the use of a surface coating to increase the surface tensionof objects.

SUMMARY

The coating of surfaces of solid objects is a widely used means ofspecifically changing the surface properties of objects. Here referenceis only made to the countless methods of corrosion protection forevery-day objects made of base metals or the painting of objects withthe aim of changing their optical appearance.

Coating is usually understood as a manufacturing process for applying afirmly adhering layer of a shapeless material to a work piece or asupporting sheet. One can in principle distinguish between four groupsof coating processes which each differ by the state of the coatingmaterial before coating:

1. Coating from a gaseous or vaporous state such as evaporation coatingor metallizing;

2. Coating from a liquid, pulpy or paste-like state such as painting,dispersion or hot-melt coating;

3. Coating from an ionized state by electrolytic or chemical depositingsuch as among others the eloxal process or electrophoretic painting.

4. Coating from a solid i.e. granular or powder state such as powdercoating or coating by sintering.

Plastics have turned out to be the production material of choice forfields of application in which cheap single-use articles are usedespecially in medical diagnostics or environmental analysis. Sinceplastics and in particular cheap, easy-to-process commodity plasticsthat can be almost shaped at will, are usually composed of non-polarorganic polymers, most of them cannot be wetted or only poorly wetted bypolar media such as water and aqueous liquids, in particular biologicalsamples. This property is utilized for certain applications especiallywhere adherence of the liquid to the plastic object is not desired suchas for disposable pipette tips. On the other hand there are numerousfields of applications for plastic objects in which a wettability withpolar liquids is desired such as for example in the field of rapiddiagnostics in which plastics can serve as materials for taking upbiological sample liquids.

There is therefore no lack of different processes which have the aim ofhydrophilically modifying plastic surfaces. Hydrophilic surfaces arecharacterized by a high surface tension which have a value near to thevalue of the surface tension of water (72 mN/m). Examples of ahydrophilic modification of plastic surfaces are corona plasmatreatment, plasma chemical vapour deposition (PACVD, e.g. Antec Co.Kelkheim), the covalent binding of hydrophilic polymers with aphotoreactive capability onto a plastic surface described for example inU.S. Pat. No. 4,973,493 (Photo Link Surface, BSI Corporation Co., EdenPrairie, Minn., U.S.A.), the application of layers containing wettingagents onto a plastic surface (for example Adhesive Research Co., GlenRock, Pa., U.S.A.) or coating of inorganic-organic nanocomposites bymeans of sol-gel technology on the surfaces to be modified (for exampleINM, Co. Saarbrucken).

The processes mentioned above which each have advantages anddisadvantages have been able to establish themselves for the hydrophilicmodification of plastic surfaces whether of foils or formed pieces.

The corona plasma treatment leads to an increase in the surface tensionwhich continuously decreases after the treatment over a short period ofseveral days up to a few weeks. Moreover the surface tensions that canbe achieved are relatively low.

Slightly higher surface tensions than those of the corona plasmatreatment can be achieved with the PACVD technology. Although thismethod leads to surface coatings that are more stable over time, adecrease of the surface tension with time is still observed in thiscase.

Surfaces which are furnished with so-called photo link surfaces or whichhave been coated with layers containing wetting agents have a surfacetension which is stable over time. However, both variants do not lead tooptimally high surface tensions. An additional fundamental disadvantageof coating with layers containing wetting agents is that the wettingagents accumulate in a liquid aqueous sample that is contacted with thelayer and can thus change it or make it unusable.

Coating of inorganic-organic nanocomposites by means of sol-geltechnology on the surfaces to be modified leads to increased surfacetensions. However, a disadvantage is that the process itself is verycomplicated and time-consuming which is why it does not appear to besuitable for modifying mass-produced plastic articles.

The object of the present invention was to eliminate the disadvantagesof the prior art. In particular it was intended to provide surfacecoatings with a high surface tension that are stable over time to beused for example for the hydrophilic modification of objects. Moreoverit should be possible to carry out the coating in a simple and reliablemanner in order to be suitable for the modification of mass-producedarticles.

The object is achieved by the subject matter of the invention ascharacterized in the patent claims.

The invention concerns a process for the production of a surface coatingto increase the surface tension of objects characterized in that a layerof at least one element that can be oxidized with water or an alloy thatcan be oxidized with water is deposited on an object and this is atleast superficially oxidized by the subsequent action of boiling wateror water vapour on the deposited layer.

The invention additionally concerns the use of a surface coating toincrease the surface tension of objects characterized in that thesurface coating is obtained by deposition of a layer of at least oneelement that can be oxidized with water or an alloy that can be oxidizedwith water and subsequent action of boiling water or water vapour on thedeposited layer.

Boiling water or water vapour and particularly preferably deionizedwater is preferably used to treat the deposited surface coating.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned aspects of exemplary embodiments will become moreapparent and will be better understood by reference to the followingdescription of the embodiments taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a top view of a portion of an analytical test element.

FIG. 2 is a view taken along lines 2-2 of FIG. 1.

DETAILED DESCRIPTION

The embodiments of the present invention described below are notintended to be exhaustive or to limit the invention to the precise formsdisclosed in the following detailed description. Rather, the embodimentsare chosen and described so that others skilled in the art mayappreciate and understand the principles and practices of the presentinvention.

The increase in the surface tension results from an increase in thepolarity and corresponds to an increased hydrophilicity of the observedsurfaces. The hydrophilicity is the water affinity of a surface. In thisconnection hydrophilic surfaces are water-attracting surfaces. Aqueoussamples also including biological samples like blood, urine, saliva,sweat and samples derived therefrom such as plasma and serum spread wellon such surfaces. Such surfaces are characterized among others in that aboundary surface of a water drop forms an acute rim or contact angle onthem (cf. for example the statements under the entry “Benetzung” in the“CD Römpp Chemie Lexikon” version 1.0, 1995). In contrast an obtuse rimangle is formed at the interface between a water drop and a surface onhydrophobic i.e. water repellent surfaces.

The rim angle which is a result of the surface tensions of the testliquid and of the surface to be examined is a measure of thehydrophilicity of a surface. Water for example has a surface tension of72 mN/m. If the value of the surface tension of the observed surface ismuch below this value i.e. more than 20 mN/m, then the wetting is poorand the resulting rim angle is obtuse. Such a surface is referred to ashydrophobic. If the surface tension approximates the value which isfound for water then the wetting is good and the rim angle is acute. If,in contrast, the surface tension is the same as or higher than that ofthe value found for water, then the drop runs and there is a totalspreading of the liquid. It is then no longer possible to measure a rimangle. Surfaces which form an acute rim angle with water drops or onwhich a total spreading of a water drop is observed are referred to ashydrophilic.

According to the invention the surface coating should be carried out bydepositing a layer of at least one element that can be oxidized withwater or an alloy that can be oxidized with water on an object. Allobjects come into consideration as objects to be coated whose surface inthe uncoated state has a lower hydrophilicity than in the coated stateafter treatment. Examples are plastic, metal, glass, ceramics, paper,fleece, cardboard etc. where the objects can have any shape e.g. flat,three-dimensional or porous.

All 4 coating processes mentioned above can in principle be used todeposit the layer i.e. coating from a gaseous or vaporous state, from aliquid, pulp or paste-like state, from an ionized state or from a solidstate. The coating is preferably carried out from a gaseous state. Alayer of an element or an alloy that can be oxidized with water isparticularly preferably applied to the surface to be modified in avacuum coater. This process is for example used cost-effectively forlarge surfaces for the packaging industry and electrical industry forexample with aluminum as the element that can be oxidized with water.

The layers can be applied to the object to be coated as a continuouslayer covering the whole area but also in the form of anytwo-dimensional patterns.

It is essential for the subject matter of the invention that thedeposited layer of the element that can be oxidized with water or alloythat can be oxidized with water is subsequently treated by the action ofboiling water or water vapour after the actual coating process. In thiscase it is sufficient to allow boiling water or water vapour to act atnormal pressure. However, it is preferably to use superheated watervapour since this considerably reduces the exposure times. For example a30 nm thick aluminum layer can be completely oxidized through withsuperheated water vapour within ca. 45 s.

In this treatment the deposited layer of the element that can beoxidized with water or the alloy that can be oxidized with water is atleast superficially oxidized. In this process at least the surface ofthe deposited layer of the element that can be oxidized with water orthe alloy that can be oxidized with water loses its elementalproperties. In the case of metals or alloys thereof this means amongothers the loss of lustre and conductivity. In this process theoxidation of the surface goes beyond the layer that may be present as anatural protective layer. Thin metallic layers are oxidized duringtreatment with boiling water or water vapour to such an extent that theycompletely lose their metallic appearance and may become completelytransparent.

The treatment with boiling water or water vapour is preferably carriedout with pure water and preferably with deionized water (for exampleaqua purificata according to DAB). The surfaces obtained in this processare not contaminated with chemicals which are found in other oxidationprocesses in particular wet chemical oxidation processes such asoxidation with oxidizing acids. Purification and rinsing steps cantherefore be omitted and make the process more simple andcost-effective.

According to the invention it is possible to use base elements for thecoating which can be oxidized by boiling water or water vapour. Theelements Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, Zr, Nb, Cd, In,Sn, Sb are preferred. Al, Si, Ti, Zr are particularly preferred. Al isquite especially preferred.

Base alloys that can be oxidized by boiling water or water vapour canalso be used according to the invention. These are preferably alloyswhich contain at least two components from the following group ofelements comprising Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, Zr,Nb, Cd, In, Sn and Sb. An alloy of 99% by weight Al and 1% by weight Siis for example suitable.

Alloys are also suitable which contain at least one component from thefollowing first group of elements comprising Al, Si, Ti, V, Cr, Mn, Fe,Co, Ni, Zn, Ga, Ge, Zr, Nb, Cd, In, Sn and Sb which are alloyed with atleast one element from the following second group of elements comprisingMg, Ca, Sr and Ba such as for example an alloy of 95% by weight Al and5% by weight Mg. Alloys are particularly preferred which contain atleast one component from the following first group of elementscomprising Al, Si, Ti and Zr which are alloyed with at least one elementof the following second group of elements comprising Mg, Ca, Sr and Ba.Alloys that are composed of aluminum which is alloyed with at least oneelement from the following group of elements comprising Mg, Ca, Sr andBa are quite especially preferred.

The deposited coat according to the invention preferably has a thicknessof 1 nm to 500 nm. The action of boiling water or water vapour on thedeposited layer leads to the formation of oxide layers which arepreferably between 0.1 nm and 500 nm, particularly preferably between 10nm and 100 nm thick.

The surface coatings produced according to the invention are homogeneousinorganic oxide layers which have a high surface tension and highpolarity, which are stable over a long period and have a good adherencewith concomitant elasticity on the coated object.

The characterization of surfaces with high surface tensions (high-energysurfaces) causes certain technical measurement problems. In variousmeasuring methods such as contact angle measurement the interfacialeffects of the surface to be examined relative to test liquids are used.Since it is hardly possible to produce test liquids with surfacetensions that are considerably above the value for water (72 mN/m),there are certain limits to being able to extrapolate above this value.Kinetic effects also add to the problems. A liquid which wets a surface,spreads over a short period typically less than 1.5 s. However,spontaneous wetting is important for the determination of the surfacetension with regard to capillary activity. In dynamic contact anglemethods one refers to the progressive angle of wetting of a drop that isbecoming continuously larger. However, the dosing of the drop andmeasurement of the drop with commercially available automated contactangle measuring instruments is too slow for the hydrophilic surfacecoatings presented here to be able to unequivocally describe the surfacecoatings according to the invention. It therefore turned out to bepractical to measure the surface coating according to the invention bycomparative determination of the filling time of a test capillary.

The cylindrical test capillary is composed of two parallel opposingfoils which are kept at a defined distance from one another by anexactly defined spacer in the form of a double-sided adhesive tape whichdetermines the length, width and height dimensions of the capillary. Theopposing surfaces of the two foils are provided with the coating to betested. The dimensions of the capillary are 0.1 mm height, 2 mm widthand 15 mm length where the length of the capillary corresponds to theavailable transport path of the liquid and the height of the capillaryis that dimension which causes the capillarity. The capillary has anopening that serves to take up the test liquid with a cross-sectionalarea of 0.1 mm.times.2 mm which corresponds to the base area of thecylindrical capillary. A vent opening is provided at the opposite end ofthe capillary which allows the displaced air in the capillary to escapewhen the test liquid is sucked in. The test liquid is preferablydistilled water or aqua purificata according to DAB but it is alsopossible to use other test liquids depending on the field of applicationof the surface coating such as blood or other body fluids. The fillingtimes for the test liquids are measured at half (7.5 mm transport path)and complete filling (15 mm transport path). Comparative results aredescribed in example 1.

The surface coatings 18 are particularly preferably used according tothe invention in analytical test elements 10 to increase thehydrophilicity. The invention therefore also concerns an analytical testelement or a test strip in which the sample liquid is transported from asample application site 12 to a determination site 14 in which adetection site lies upstream of the sample application site in thetransport direction. In this connection it is important that theanalytical test element 10 has at least one surface 16 which is composedof at least one element that can be oxidized with water or an alloy thatcan be oxidized with water which has been treated by the action ofboiling water or water vapour. The materials described above come intoconsideration as elements and alloys.

Surfaces that have been made hydrophilic are especially essential forcapillary gap test elements. Capillary gap test elements are testelements in which the sample liquid is moved in a transport channel(capillary channel, capillary gap) from a sample application site to adistant sample detection site with the aid of capillary forces in orderto undergo a detection reaction at this site.

The ability of a capillary to suck up a liquid depends on thewettability of the channel surface with the liquid. This means foraqueous samples that a capillary should be manufactured from a materialwhose surface tension almost reaches 72 mN/m or exceeds this value.

Sufficiently hydrophilic materials for the construction of a capillarywhich rapidly sucks up aqueous samples are for example glass, metal orceramics. However, these materials are unsuitable for use in testcarriers since they have some disadvantages such as risk of breaking inthe case of glass or ceramics. Therefore plastic foils or moulded partsare usually used to manufacture test elements. As a rule the plasticsused hardly exceed a surface tension of 45 mN/m. Even with the, in arelative sense, most hydrophilic plastics such as polymethylmethacrylate(PMMA) or polyamide (PA) it is only possible—if at all—to constructslowly sucking capillaries. Capillaries made of hydrophobic plasticssuch as for example polystyrene (PS), polypropylene (PP) or polyethylene(PE) essentially do not suck aqueous samples. Consequently it isnecessary to endow the plastics used as a construction material for testelements with capillary active channels with hydrophilic properties i.e.to hydrophilize them.

Therefore in analytical test elements with capillary gaps at least one,but preferably two and especially preferably two opposing surfaces whichform the inner surface of the channel capable of capillary liquidtransport are preferably hydrophilized. If more than one surface ishydrophilized then the surfaces can either be made hydrophilic using thesame or different methods. Hydrophilization is particularly necessarywhen the materials that form the capillary active channel are themselveshydrophobic or only very slightly hydrophilic because they are forexample composed of nonpolar plastics. Nonpolar plastics such as forexample polystyrene (PS), polyethylene (PE), polyethylene terephthalate(PET) or polyvinyl chloride (PVC) are advantageous as carrier materialsbecause they do not absorb the liquids to be examined and thus thesample volume can be effectively utilized by the detection reaction. Thehydrophilization of the surface of the capillary channel enables apolar, preferably aqueous, sample liquid to readily enter the capillarychannel and be rapidly transported there to the site of the test elementwhere the detection takes place.

The hydrophilization is quite especially preferably achieved by usingthin layers of oxidized aluminum. These layers are either applieddirectly to the desired components of the test element for example byvacuum coating the work pieces with metallic aluminum and subsequentlyoxidizing the metal, or by using metal foils or metal-coated plasticsfor the construction of the test carriers which also have to be oxidizedto achieve the desired hydrophilicity. In this case metal layerthicknesses of 1 to 500 nm are adequate. The metal layer is subsequentlyoxidized to form the oxidized form whereby according to the inventionoxidation in the presence of water vapour or by boiling in water haveproven to be especially suitable methods. The oxide layers formed inthis manner are between 0.1 and 500 nm, preferably between 10 and 100 nmthick depending on the method. Larger layer thicknesses of the metallayer as well as of the oxide layer can in principle be realized inpractice but do not exhibit any additional advantageous effects.

Further applications according to the invention of surface coatings are:

-   -   making polymer foils, fabrics, fleeces or three-dimensional        formed pieces hydrophilic and detergent-free to improve their        adhesive properties for paint, adhesive or plastic coats,    -   adsorptive binding of polar molecules or materials in particular        for the adsorption of biologically active molecules such as        proteins, enzymes, antibodies, nucleic acids (cf for example        Chem. Pharm. Bull. 41, (1993) 1055 since the increased surface        tension is associated with an increase in the polarity of the        surface,    -   production of surface patterns such as cross-pieces for the        transport of polar liquids, for a defined boundary between polar        and nonpolar areas in which case these patterns can be produced        by the structure of the deposited material itself or        subsequently by selectively covering certain areas of the        deposited material layer with for example wax when it is treated        with boiling water or water vapour.

The invention is elucidated by the following example.

Example 1 Comparison of the Filling Times of a Test Capillary

A cylindrical test capillary was constructed from two parallel opposingfoils which are kept at a defined distance from one another by anexactly defined spacer in the form of a double-sided adhesive tape whichdetermines the length, width and height dimensions of the capillary. Theopposing surfaces of the two foils were provided with the coating to betested. The foils that were used were polyester foils (MELINEX®, ICI)which were variously modified as follows.

1.) Foil which was coated with photoreactively equipped hydrophilicpolymers (BSI)

2.) Foil which was subjected to a corona plasma treatment (Antec)

3.) Foil on which a layer containing a wetting agent was applied (A.R.)

4.) Foil on which inorganic-organic nanocomposites were coated by meansof sol-gel technology (INM)

5.) Foil on which aluminum oxide was vapour deposited but which was notsubsequently treated with boiling water or water vapour (Alu-Ox).

6.) Foil on which firstly a layer of aluminum of 30 nm thickness wasvapour deposited which was subsequently completely oxidized for 45 swith water vapour (Alu-Ox/H₂O).

The dimensions of the capillary are 0.1 mm height, 2 mm width and 15 mmlength where the length corresponds to the transport path of the liquidand the height is that dimension which causes the capillarity. Thecapillary has an opening that serves to take up the test liquid with across-sectional area of 0.1 mm.times.2 mm which corresponds to the basearea of the cylindrical capillary. A vent opening is provided at theopposite end of the capillary which allows the displaced air in thecapillary to escape when the test liquid is sucked in. The test liquidwas in one case distilled water, aqua purificata according to DAB and inthe other case human blood (containing EDTA as an anticoagulant,haematocrit 44%). The filling times for the test liquids were measuredat half (7.5 mm transport path) and complete filling (15 mm transportpath). The capillary was kept vertical during filling so that the testliquid had to cover the filling path of 15 mm against the force ofgravity. The comparative results are shown in Table 1.

TABLE 1 Filling time for blood[s] Filling time for water[s] Coatingafter 7.5 mm after 15 mm after 15 mm BSI 0.5-0.6 2.1-2.7 0.6-0.9 Antec1.4-2.3 4.7-8.4 4.0-7.0 A.R. 0.4-0.5 1.8-2.1 0.4-0.5 INM 1.0-3.0 Alu-Ox— 1.2 — Alu-Ox/H₂O 0.2-0.3 0.7 <0.2

The coating according to the invention (Alu-Ox/H₂O) is the one whichwets most actively among all tested layers. The coating according to theinvention (Alu-Ox/H₂O) produced by the action of water vapour aftercoating the metal has a considerably improved hydrophilicity compared toa surface (Alu-Ox) that was coated directly with aluminum oxide but wasnot oxidized by the action of water vapour. The production processaccording to the invention is relatively simple, can be controlled welland leads to stable coatings with high surface tensions that remainconstant over time.

While exemplary embodiments incorporating the principles of the presentinvention have been disclosed hereinabove, the present invention is notlimited to the disclosed embodiments. Instead, this application isintended to cover any variations, uses, or adaptations of the inventionusing its general principles. Further, this application is intended tocover such departures from the present disclosure as come within knownor customary practice in the art to which this invention pertains andwhich fall within the limits of the appended claims.

1. An analytical test element, comprising: a sample application site; adetection site disposed upstream of the sample application site in asample transport direction; and at least one surface formed from atleast one element or alloy that has been oxidized by boiling water orwater vapor, the surface comprising an oxide of the element or alloy. 2.The test element of claim 1 wherein the element is derived from at leastone element selected from the group consisting of Al, Si, Ti, V, Cr, Mn,Fe, Co, Ni, Zn, Ga, Ge, Zr, Nb, Cd, In, Sn, Sb.
 3. The test element ofclaim 1 wherein the element is derived from at least one elementselected from the group consisting of Al, Si, Ti, Zr.
 4. The testelement of claim 1 wherein the element is Al and the oxide is aluminumoxide.
 5. The test element of claim 1 wherein the alloy contains atleast two components selected from the group of elements consisting ofAl, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, Zr, Nb, Cd, In, Sn, Sb.6. The test element of claim 1 wherein the alloy contains at least onecomponent selected from a first group of elements consisting of Al, Si,Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, Zr, Nb, Cd, In, Sn, Sb, which arealloyed with at least one element selected from a second group ofelements consisting of Mg, Ca, Sr, Ba.
 7. The test element of claim 6wherein the alloy contains at least one component selected from a firstgroup consisting of Al, Si, Ti, Zr, which is alloyed with at least oneelement selected from a second group consisting of Mg, Ca, Sr, Ba. 8.The test element of claim 7 wherein the alloy is composed of Al which isalloyed with at least one element selected from the group consisting ofMg, Ca, Sr, Ba.
 9. The test element of claim 1 wherein the at least onesurface has a thickness between 1 nm and 500 nm.
 10. The test element ofclaim 1 wherein the surface comprises a superficial oxide layer having athickness of between 0.1 nm and 500 nm.
 11. The test element of claim 10wherein the surface comprises a superficial oxide layer having athickness of between 10 nm and 100 nm.
 12. The test element of claim 1,further comprising a capillary between the sample application site andthe detection site, the capillary comprising the at least one surfacecomprising the oxide of the element or alloy.
 13. The test element ofclaim 12, wherein the oxide comprises aluminum oxide.
 14. The testelement of claim 12, wherein the oxide comprises a layer covering aplastic substrate.